Bussard Polywell Fusion Advances

Next Big future blog has been working hard at keeping on top of developments around the Bussard Polywell.I have tacked together the last three posts and added some of the pictures.We pretty well know how big the reactor sphere has to be and it looks like technical issues are getting sorted out.

The sphere needs to be at least 1.3 meters in radius.More importantly, a small increase above that will have a dramatic effect on energy production.Thus if you can achieve break even the next step could be already at hand.

The technology will be tested within the next twelve months.A production model will follow on its heels and full production right after that.The point been made is that production is immediate and quickly implemented.We are not facing gross regulatory barriers as with hot nuclear.

The project has been satisfactory and encouraging to the present.It really looks like it may simply work.It also looks easily put into mass production and will outright replace all other energy systems in two decades.It is also amenable to a rapid reduction in cost as manufacturing ramps up.

This tech has the potential to trade out all energy technology and do it cheaply and safely and even quickly.We will continue to follow it and its sister technology at Focus Fusion (they are both small).

You won't hear Rick Nebel talking about fusion as a challenge requiring billions of dollars and decades of experimentation. For the past couple of years, Nebel heads up a handful of researchers following the less-traveled path to fusion at EMC2 Fusion Development Corp. in Santa Fe, N.M. That path involves creating a high-voltage chamber to sling ions so energetically at each other that at least some of them fuse and release energy.

EMC2 recently created a buzz in the fusion underground by reporting on its Web site that a series of experiments was able to "validate and extend" earlier results reported by the late physicist Robert Bussard. The company is now using a $7.9 million contract from the U.S. Navy to build a bigger test machine, known as WB-8. (WB stands for "Wiffle Ball," which refers to the shape of the machine's magnetic fields.)

Success isn't assured. The WB-8 experiment could conceivably show that the approach pioneered by Bussard, known as inertial electrostatic confinement fusion or IEC fusion, can't be scaled up to produce more power than it consumes. And if Nebel's team comes to that conclusion, he doesn't plan to pull any punches.

"No B.S. and no excuses," Nebel told me over the weekend. "If it looks like we have a problem with this, we're going to tell them."

But if IEC fusion actually works, Nebel wants to be ready to commercialize the technology. "Generally what you want to do is have one machine operating, one machine being built, and one machine designed," he said. "We want to be in a position that if we have good results from WB-8, we can hit the ground running."

That's what the contributions being sought under the umbrella of the New Mexico Community Foundation would go toward, he said. Nebel doesn't expect anything near $200 million to start with. "We're just looking for a few hundred thousand to do the design work and do some basic physics on this," he said. "There are some open questions we have to take a look at."

The EMC2 Fusion Web site sports a picture of a 100-megawatt WB-D fusion demonstrator, which looks like a cube about 20 feet on a side. Nebel said the eventual design may not look like the picture, but he does believe the best path to success leads to relatively small-scale reactors rather than the mega-reactors envisioned by ITER's backers.

"The key to making any of these things attractive is being able to make them small," he said.

Nebel can't yet predict whether his path will pan out. Some experts say the equations of plasma physics suggest that Wiffle Ball devices can never produce more power than it consumes, and that IEC research is destined to lead to a dead end. But so far, Nebel sees no reason to stop moving ahead. "It's been quite a trip on this thing," he said, "and I have a feeling this is going to continue."

It is noted that a producing polywell needs to have a radius in excess of 1.3 m.

Bussard's landmark 2006 publication showed that the power balance (Q) increases with the size of the Polywell machine as the 5th power of the magnet radius. Break-even radius is, by definition, the radius (R) of the smallest machine that produces more power than it consumes. Practical power machines must be larger than this size, but not much larger because of the steep rise of Q with R. Particle-in-cell simulation was used to find the maximum Q for each R by searching the steady-state parameter space defined at startup by knob values. Applying the 5th power scaling law to this optimum Q predicts that the break-even radius for DD fueled Polywell will be 1.3m. This is much smaller than the radius of the planned ITER design, giving Polywell an advantage over the competing magnetic confinement power generation.

Results of the experimental and analytical work conducted during the program now ending have shown all of the conclusions necessary to support and define the next step to full-scale net fusion power demonstration. These include:

1. No closed box machine can ever yield net fusion power; open recirculating MG machines and systems are required. This is an immutable result of the determination of losses of electrons in experiments, that show that losses to surfaces that are NOT magnetically shielded must be kept to less than 1E-5 or so of the cusp axis flow of electrons in the WB effect at beta = one. This is impossible for two reasons: (a) it is not practically possible to cover all but 1E-5 of the entire surface of a box containing the interior plasma, with magnetic oils that protect all of this surface, and (b) even if this were possible, it is not possible to protect against losses directly along the cusp axes to the end plates that bound each cusp. These intrinsic losses are inherent in the magnetic topology of a closed box system and forever prevent this from operating at small losses.

2. The inescapable conclusion is that all polyhedral Polywell® machines must operate as open recirculating devices, and that all such systems must have essentially no B-field unshielded surface area available to electrons in the machine, itself. This means that all structure containing B field-generating coils must be conformal to the fields so produced, thus coil containers must have elliptical or circular cross-sections. If not, there will be large regions in which B fields go into the metal surfaces at an angle rather than circulate around such surfaces. And electrons will simply drive along these intersecting B fields, directly into the metal, to yield excessive losses.

3. Because of this, it is also evident that – no matter their individual plan form shape (i.e. circles, squares, triangles, polygons etc.) – magnet coils must not touch at their adjacent corners, but must be spaced sufficiently far apart to ensure that no B fields intersect their containers. In this way, electrons can recirculate freely around all parts of each coil, and thus operate with minimal losses. These corner spacing line-like-cusps give local current flows that reduce the effective e- trapping factor (Gmj) in the machine interior from that for pure WB behaviour alone. However, the reduction is not sufficient to prevent ready attainment of the e- density ratios inside/outside, required for avoidance of external arcing (see below, in Conclusions 4, 7, 8)).

4. Operating as recirculating (MaGrid) machines means that there will be an external region between the machine and its containing exterior wall, in which Paschen arc breakdown can occur, unless both external electron and neutral gas density can be kept below some critical level. To do so requires large scale vacuum pumping in this exterior region. However, this level is so low that it can not produce significant fusion rates inside the machine, if the densities are allowed to be the same across the system. Thus, some means must be found to ensure large electron density within the machine, while maintaining it at small levels outside.

5. This requires that the ionization (of neutral gas) density within the machine be very large relative to that outside; and this can be attained only by neutral gas injection directly into the machine, followed by subsequent very rapid ionization of this gas, before it can escape into the exterior region. In small machines this is difficult, as time scales for neutral transport to the exterior are measured in fractions of a millisecond, and dimensions within the machines are not sufficient to allow rapid ionization at the limited electron currents and densities attainable. In large machines, such as power reactors (typically 2-3 m in diameter) with high power electron drives (e.g. 100-500 Amps at 15- 30 kV for DD and 180-220 kV for pB11), it is easy to show that almost total ionization of inflowing neutral gas can be achieved in a few cm of electron path length at the system edge, but small devices can not reach this condition.

6. Thus, in small systems there is a big incentive to attempt to fuel the machine with ions injected from ion guns placed on cusp axes. This, however, poses the problem that the ion guns must be at machine voltage, thus constitute very visible and attractive potential sinks for electrons, as they can not be fully magnetically shielded, as can the magnets themselves. In this situation, it appears that the only way to test these principles in small machines is to try to use capacitor discharge drives, timed precisely so that neutral gas injection is started with the cap drives, and the electron well drives are also started simultaneously. This requires very precise timing, which is difficult but has been achieved in such tests, however, this entire problem goes away in machine sizes for net power production. This conclusion echoes that of previous years. If it were possible to provide ion injection surfaces on the inside faces of the magnets (but no such sources exist), this might solve the problem in small test devices, however, ions injected at low energy at such positions will, themselves, be trapped in the magnet surface B fields, and have to cross into the potential well gradient by ExB drift forces, which may not be practicable. In reactor-size systems, ions formed within the interior field surface boundary will fall to the center naturally, under the effect of the high radial potential gradient that makes the deep well of the system.

7. Finally, in terms of practical limitations it was noted that the basic physics concept presumes magnet coils of near-zero physical cross-section, which touch at acute to right angles at the corners of the polyhedral-vertex boundaries on which they are supposed to lie. This has always given a “funny cusp” at such touching corners, which has been noted as having essentially zero tangential radius, although it also has zero B field. However, with realistic coils of finite dimensions (i.e. the coil cross-sections are a not insignificant fraction of the machine or coil major radius) this “funny cusp” expands to involve a rectangular region bounded by the dimensions/size of the coil containers. This rectangular region will have competing fields at 90 degree intervals, thus will act as an unshielded area for electron losses from the machine drive. The fractional size of this unshielded area is always found (from magnet design studies using real conductors) to be in the range of 0.01-0.1 of the total surface area of the coil containers. Since unshielded fractional areas above 1E-5 to 1E-4 are untenable, this effect gives losses that are ca. 1000x too large for useful fusion output.

8. The only way to avoid this, with coils of realistic finite size, using realistic conductors (e.g. superconductors) is to space the coils a distance from each other, as described in (3), above, so that NO B fields intersect the coil container metal surfaces, but rather the field lines flow in parallel between the spacing at these corners. To achieve the ideal polyhedral trapping effect with proper coil magnetic insulation, the coil centerlines may also be offset so as to appear directly along the edge vertices,. although this is not an essential requirement. Thus, the only coil configuration that can work to best advantage is one in which the coils are contained in circular cross-section tubes, turning at each corner through a small straight section, which is spaced a distance away from its not quite- touching adjacent neighbor coil. Analysis shows that this spacing should be at least 3-8 gyro radii of the electrons in the coil surface field. This will avoid all direct incident electron impact but, as noted previously, will result in increased electron flow between inside and outside due to the fact that the spaced regions act like small line cusps rather than point cusps. Greater coil spacing can be used but only at the price of lesser internal trapping. A balance must be struck between Paschen arcing exterior density, and interior density required for the desired fusion output. Fortunately, it has been found that a margin of about 1000x is available in design for these conditions.

9. These line cusp flow increases will operate in parallel with the cusp-confinement Gwb of the basic coil geometry, and will thus reduce the overall trapping factor to something less than Gwb. Calling the overall trapping factor Gmj, it is found that Gmj can be computed as the inverse sum of the two trapping factors for the machine; one being Gwb, the other being the line cusp factor Glc, weighted by the fractional area “seen” by electrons for each type of loss. Thus 1/Gmj = (1-flc)/Gwb + flc/Glc, where flc is the fractional line cusp area in the system. These loss mechanisms act as parallel flow channel factors. If the line cusp corner dimensions are only a few cm, the reduction in effective trapping from the basic Gwb may still be a factor of 2-5x.

10. This has the consequence that the maximum electron density ratio that can be sustained between inside and outside will be equally reduced, and the outside density must be that much larger for a given interior density (as required for useful fusion output). This requires greater vacuum pumping in the exterior, to reduce ionization from the higher background density, and limits the ability of small systems still further to be run (even for very short times) in the capacitor-drive pulsed mode. Since Gmj factors needed to avoid Paschen arcing are in the range of 1E3-1E4, while basic Gwb factors are one or two orders of magnitude larger than this, the avoidance of arcing at fusion conditions in the interior is easily attainable even with the spaced corner flow increases.

11. Once again, large machines will not suffer from these problems to any significant degree, but they will cost a great deal more. Costs tend to scale as the cube of the system size and the square of the B field. Thus, full-scale machines and their development will cost in the range of ca $ 180 – 200 M, depending on the fuel combination selected. These cost estimates closely reproduce those made throughout the USN program life, from its earliest work (1991) to its conclusion (mid-2006) including those made at interim reviews (1995, 1999). US Navy costs expended to date in this program have been approximately $18 M over about 10 years (2/3 in last 6 years).